Revetment blocks are available in a variety of shapes and sizes for use in different applications. The erosion of ground surfaces can be controlled by installing heavy revetment blocks on the surface, generally on top of a geotextile filter fabric. The areas to be protected are those where there is substantial water flow, with a significant velocity. This includes watershed areas, channels, spillways, etc., where there either is a continuous flow of water, a periodic flow of water due to heavy rains or floods, or other areas where any erosion of the soil would be undesirable and detrimental.
The areas to be protected from erosion are generally graded to make the surfaces level, and then compacted. The graded surfaces can be inclined, such on the sides of a river bank or water shed area, or level such on the bed of a river or other area. Sheets or rolls of the geotextile material are then laid on the graded surface. Lastly, the revetment blocks are installed on the geotextile material either by hand, or in 8 foot up to 40 foot mats cabled together, depending on the specifications required of the project. The revetment blocks can be of the positive interlocking type such as disclosed in U.S. Pat. Nos. 5,556,228; 6,955,500; 8,123,434 and 8,123,435. Some of the blocks known in the prior art are shaped so that a portion of one block overlies a portion of the neighbor block to provide stability. The disadvantage with this type of block is that if one block breaks and requires replacement, one or more of the neighbor blocks are required to be manually lifted so that a portion of the replacement block can be slipped thereunder. Other blocks are positive interlocking, such as described in U.S. Pat. No. 5,556,228, so that each block can be lifted vertically and removed from the matrix without disturbing the neighbor blocks.
The effectiveness of revetment blocks to control the erosion of the underlying soil is a primary concern to those who select the type of block to be used. Because of the shape, weight and cost of revetment blocks, some blocks can better protect the underlying soil than other blocks. Another factor to take into consideration in the selection of a revetment block for a particular application, is the safety factor. The safety factor relates to the ability of the revetment block to withstand high velocity of water without lifting due to hydraulic forces. A matrix of revetment blocks having a smooth top surface has a high safety factor, as there are no frontal edges that the high velocity water can abut against and cause a lifting force to be exerted on the block. The water abutting against any frontal vertical edge of a revetment block exerts a lifting force as the water hits the frontal edge and rises to run over the top of the block. If the revetment block experiences a sufficient lifting force, it can be lifted out of place in the matrix and carried downstream. When this occurs, the integrity of the matrix is compromised, whereupon the neighbor blocks are more easily lifted and removed from the matrix also. Depending on the velocity and depth of the water flowing over the matrix, this chain reaction can continue until many or all of the revetment blocks are carried downstream. The erosion of the underlying soil is then unabated.
The safety factor of revetment blocks can be calculated by well known formulas which take into consideration the expected maximum velocity of water, the depth of water flowing over the matrix of revetment blocks, the grade or angle on which the revetment blocks are installed, the extent of disruption between neighbor blocks (surface unevenness), etc. The safety factor can be a numerical value between zero and upwardly to ten, or higher. A safety factor of zero means that there is a very high probability that the block will fail, and a high safety factor means that there is a low likelihood that the block will fail when subjected to worst case water flow. Generally, a minimum safety factor of about 1.9-2.0 is acceptable to most installation specifications to minimize liability and provide a sufficient degree of comfort that a mat or matrix of blocks will perform as expected based on the worst case conditions.
An acceptable safety factor can generally be achieved even with a vertical mismatch between neighbor blocks of about 0.5 inch. In other words, in order for a revetment block to achieve an acceptable safety factor, the vertical difference in height between any adjacent connected block cannot be more than 0.5 inch. If this is the worst case condition, the water will abut against the frontal edge of the downstream block and exert a lifting force, but the block will remain in place. A difference in the height of the side edges on the neighbor blocks is also a concern, as the hydraulic lifting force thereon is also exposed due to the uneven edges. Even when all of the revetment blocks are constructed with uniform thicknesses, the top surfaces or edges thereof may not provide a uniform smooth surface for the matrix. This can occur when the underlying soil is not sufficiently smooth, which is often the case. Even though the soil is graded and made as smooth as possible prior to the installation of the revetment blocks thereon, in practice there may be irregularities and undulations in the surface of the soil. Even if the soil is prepared with a very smooth surface and the blocks installed thereon, after a period of time, the soil can settle or fine soil particles can be carried away, thereby leaving an irregular surface on which the blocks rest. Revetment blocks are often used in application where the soil is characterized as being dispersive, or where differential settlement occurs. Such applications include landfills and coal mines. With regard to landfills, as the materials compost and otherwise deteriorate and degrade, the soil collapses and results in nonuniform surfaces. Coal mines are filled with earth material after being mined out, thus allowing areas to settle in a nonuniform manner. Thus, even if the revetment blocks are installed so that the surface underlying each block is even, this situation does not often remain over time. Accordingly, as time goes on, the block edges become uneven and the safety factor is effectively lowered.
From the foregoing, it can be seen that a need exists for a revetment block that exhibits an acceptable safety factor even when edge differences approach the industry standard of 0.5 inch. Another need exists for a revetment block that includes a purposeful difference in the height between neighbor block edges, at the interface between upstream and downstream edges, as well as the side-to-side edges. The frontal edge of the downstream block is made thinner than the back edge of the upstream block. Yet another need exists for a revetment block that is positive interlocking, i.e., having arms and sockets, and is constructed with a built-in mechanism that allows differences in surface irregularities to exist without adversely affecting the safety factor.